Electrical control system



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ELECTRICAL CONTROL SYS'IEU Filed July 13, 1945 5 Sheets-Sheet 5 pear issanaa Treouczr v.5

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Patented Aug. 17, 1948 ELECTRICAL CONTROL SYSTEM Paul T. Nims, Detroit, Mich., assignor to Chrysler Corporation, Highland Park, Mich, a corporation of Delaware Application July 13, 1946, Serial No. 683,294

19 Claims.

The invention relates to electrical control systems and is particularly directed toward an improved control apparatus for regulating the transfer 01 power from an electrical input system to an electrical output system oi the type shown. described, and claimed in my copending application Serial No. 565,955, filed November 30, 1944, for Electrical control system, and assigned to the same assignee as the present invention.

The principal objects of the present invention are to provide a system of the aforesaid type which is simple in arrangement, requires a minimum number of structural elements, is relatively light in weight, and is reliable and eflicient in operation: to provide such a system in which multiphase alternating current input energy is translated into multiphase alternating current output energy; to provide such a system embodying improved means for timing the operations of the control apparatus associated with the several output phases; to provide in such a timing means a mechanical means whereby the relative phase angle between the various output phases may be fixedly predetermined; to provide such a system in which the multiphase output circuits of the several units may be connected in parallel and embodying improved means for controlling the division of load between the parallelly connected systems; to provide such a load-controlling mechanism which is operable to control the divisions of load by a relative shift in phase angles between the output of the two parallelly connected systems; and generally to improve and simplify the construction and arrangement systems of the above generally indicated type.

With the above, as well as other and more detailed objects in view, which appear in the following description and in the appended claims, preferred but illustrative embodiments of the invention are shown in the accompanying drawings, throughout the several views of which corresponding reference characters are used to designate corresponding parts and in which Figs. 1A and 1B collectively provide a diagrammatic arrangement of power and control circuits for two units arranged in parallel to supply a multiphase output circuit. In reading the drawings Fig. 13 may be placed immediately to the right of Fig. 1A so that, with this arrangement, unconnected terminals of the two sheets will line up with correspondingly designated unconnected terminals on the other sheet, thereby completing the circuits which extend from one sheet to another;

Fig. 2 is a view showing a modified control cir- 2 cuit adapted for use with the power circuit shown in Figs. 1A and 13;

Fig. 3 is a schematic arrangement of a modifled form of control circuit adapted for use with the power circuit shown in Figs. 1A and 113;

Fig. 4 is a curve depicting the flux in the inducing mechanisms shown in connection with the control circuits of Figs. 1A, 1B and 3;

Fig. 5 is a curve depicting the voltage induced in accordance with the flux curve of Fig. 4;

Fig. 6 is a diagrammatic view of the pole pieces by which the flux waves and voltage waves of Figs. 4 and 5 may be obtained;

Fig. 7 is a curve depicting the change in current through the tuned circuit of Fig. 3 with change in frequency of the voltage applied thereto; and

Fig. 8 is a series 01. curves depicting various operating characteristics of the speed-controlling mechanism depicted in Fig. 2.

Referring first to Figs. 1A and 1B, which show substantially identical power and control circuits for two substantially identical units connected in parallel to a common output circuit,

' the input circuits for the substantially identical units oi Figs. 1A and 1B are shown to be from two separate sources. However, it is to be distinctly understood that, if desired, the units could both be connected to the same common source for feeding the same common output system. For these reasons it is believed a. description of one power and one control unit will sufiice for a description of both, except in the respects hereinafter noted.

In Fig. 1A, power is supplied from a suitable source to conductors I, 2, and 3 by a suitable three-phase transformer 4 and transferred by means of a plurality of networks 6, 8, and Ill to a three-phase output system comprising conductors l2, l4, and i6. Each network 6, 8, and [0 comprises two sets of electric valve means, each set comprising three triodes Al, A2, and A3 and three triodes Bl, B2, and B3, Each of the A and B triodes have anodes, grids, and cathodes. The anodes oi the A triodes are connected respectively to lines I, 2, and 3, and the cathodes thereof are connected to a common cathode output bus 18 connected to one terminal of the primary coil of transformer T5. Likewise, the anodes of the B triodes are connected respectively to the input conductors I, 2, and 3, and the cathodes thereo are connected to a common output bus 24 which is connected to the other terminal of the primary coil of transformer T6. The primary coil of transformer T6 has a center tap which is connected by means of conductor 36 and an inductive reactor 82 to a bus connected to the center or neutral point of the Y-connected three-phase transformer 4. One terminal of the secondary coil of transformer T6 is connected directly to ground, while the other terminal is connected by conductor 42 to the output conductor I2. All the grids of the A valves oi group 6 are connected through grid resistor RI to a common grid connection 44. Grid bias voltage is supplied by a voltage inducing mechanism generally designated 48 and having a rotating permanent-magnet type rotor 49 for inducing a voltage in its output coils 46 and 41. The coil 46 has a resistance R3 in series therewith and is connected between the buses I8 and 44 whereby the grid bias of all of the grids of the A valves of group 6 may be simultaneously biased. Likewise, all the grids of the B triodes are connected through resistors R2 to a common grid bus 64 and are simultaneously biased with respect to their cathodes by the voltage induced in coil 41 which is connected between the buses 24 and 64 and has a resistor R4 in series therewith. The rotor 49 is rotated by a shaft 62 at the same R. P. M. as the cycles per minute output of the secondary coil of transformer T6 and alternately biases the grids of the A and B triodes as groups to render them in a conductive condition. In order that only one set of triodes A or B may conduct at one time, a condenser C6 is connected between the common cathode buses I8 and 24, the operation of which will be set forth in greater detail hereinafter. Condensers C8 and CID are likewise respectively connected between the common cathode buses of the A and B triodes of groups 8 and I8.

As shown in Fig. 6, the shape of the rotor 48 is tapered and arranged with respect to the taper of the pole pieces 58 and 68 of the core 6| having the coils 46 and 41. so that a flux wave similar to that shown in Fig. 4 is induced in the core BI. The coils 48 and 41 will have induced therein a voltage of substantially the form shown in Fig. 5. The provision of the square voltage wave is desirable so that the grids of the valve will be biased immediately and completely at the desired times. It is to be distinctly understood, however, that other types of voltage induction apparatus can be used to supply the bias voltage of the square wave form. Other forms of voltage waves which will place the critical bias on the grid at the correct times may be used; however, the square wave seems best suited for this purpose.

As the triodes A of group 6 are rendered conductive, current flows, from one of the input conductors I, 2, or 3, through whichever one of the A triodes AI, A2, or A3 happens to have its anode at the highest positive potential as determined by the instantaneous voltages being induced in the conductors I', 2, or 3 during the time interval that the A triodes are conductive as is more fully described in my said copending application. Current from the A triodes of group 6 flows from bus I8 through the left-hand half of the primary coil of transformer T6 back through the conductor 30, and the inductive reactance 32 to the neutral line or bus 34 causing a voltage to be induced in the secondary coil whereby current may flow in the conductor 42 and the output conductor I2. Similarly, when the B triodes of group 6 are conducting, current will flow therethrough, through bus 24 and the right-hand side of the primary coil of transformer T6 and back through conductor 38 and reactance 32 to bus 34 causing a voltage, of opposite polarity to that caused by conduction f 4 the A triodes, to be induced in the secondary coil of transformer T6.

The valve group 8 has A and B triodes havin their anodes connected to the conductors I, 2, and 3 and their cathodes connected respectively to opposite end terminals of an output transformer T8 by buses 63 and 64. The grids of the triodm of group 8 are periodically biased with re spect to their cathodes by a voltage inducing mechanism 48A which is similar in all respects to mechanism 48, except that its rotor 49A is displaced about the shaft 62 behind the rotor 48. The voltages induced in the secondary coil of transformer T8 and delivered by conductor 66 to the output conductor I4 are therefore 120 electrical degrees behind those in conductor I2. Similarly, the valve group III has A and B triodes having their anodes connected to the conductors I, 2, and 3 and their cathodes connected respectively to the opposite end terminals of an output transformer TIB by buses 65 and 61. I The grids of the triodes of this group II) are periodically biased with respect to their respective cathodes by a voltage inducin mechanism 463 similar in all respects to mechanisms 48 and 48A, except that its rotor 49B is displaced 120 about the shaft 62 behind the rotor 49A so that the voltages induced in the secondary coil of transformer TH) and delivered through conductor 68 to the output conductor I6 are 120 electrical degrees behind those in conductor I4. The center taps of the primary coils of transformers T8 and Till are respectively connected through inductive reactors I8 and I2 by means of conductors l4 and 16 respectively to bus 34. In order that the system may be disconnected from the output conductors a three-pole line switch LS-I is used to connect the conductors 42, 66 and 68 to the conductors I2, l4, and I6.

All the rotor or rotating members 49, 49A, and 48B are mechanically secured to and rotated by the shaft 62 which is driven by a direct-current motor I8. The speed of the motor 18 is controlled by means of a governor generally designated I9 which acts to open and close a shunt circuit around a current-controlling resistor RI5 which will cause the current through the field coil of the motor 18 to increase or decrease. A flux-inducing member or rotor 86 of a three-phase alternator 82 having phase coils 84, 86, and 88, is also rotatable by shaft 62. The coils 84, 86, and 88 are preferably Y connected and adapted to be connected to a similar alternator 82 of the system of Fig. 1B having a rotor 88 which is rotated by a shaft 62. The mechanisms 48, 48A, and 48B of Fig. 1B are rotated by a shaft 62 corresponding thereto and are in the same identical phase relationship as are the rotor 88 and the mechanisms 48, 46A, and 48B of Fig. 1A so that when the output voltages of the two altcrnators 82 are in phase the power systems are in phase. A three-pole switch 96 is connected to the phase coils 84, 86, and 88 of alternator 82 and when closed connect the alternator 82 to the conductors 9D, 92, and 94 respectively. Switch 96 of Fig. 1B similarly connects the alternator 82 of Fig. 13 to the conductors 90, 92, and 94. Incandescent lamp bulbs L are shunt connected with respect to the switch blades of the three-pole switch 96, so that when the systems of Figs. 1A and 1B are in proper phase relation with each other the switches 96 and LS-I may be closed. If desired, the switches 96 and LS-I may be mechanically interlocked for simultaneous operation. The relative torques exerted on the shaft 62 by the motor I8 and alternator 82 are such that the shaft is primarily rotated by the motor I8 but the torque oi the alternator 82 is operable to cause momentary changes in the angular velocity of the shaft 62 whereby the time instants, at which corresponding valves 01' the systems of Figs. 1A and 1B are rendered conductive, may be controlled.

A three-phase current transformer I is connected in series with the output conductors 42, B6, and 08 whereby the current flowing through each of these conductors 42, B, and 68 causes a proportionate current flow in each of the transformer secondary output conductors I02, I04, and I06. These secondary output conductors are connected to a full-wave three-phase rectifying device I08 having D. 0. output conductors I I0 and I I2. A resistor I I 4 is connected between the output conductors I I0 and I I2 and the voltage thereacross is proportional to the current output of the conductors 42, 66 and B8. The-terminal of the resistor II4 connected to the conductor H2 is grounded While the other terminal of the resistor I I4 is connected by conductor I I6 to an adjustable controlling element In of a D. C. motor speed control generally designated Illl which controls the motor 78. Likewise. the output conductors of Fig. 13 have current transformers therein feeding a rectifying device, a resistor II4 and an output conductor IIB connected to a controlling element II! of a D. 0. motor speed control M8 for the motor I8. The other terminals of the elements II! are connected together by a conductor II9 having disconnect switches Ilila which may be mechanically interlocked with LSI if desired. Any unbalance in the voltage drops across the two resistors I I4, causes current flow through the elements I I7, tending to balance the loading of the two systems.

Returning again to Fig. 1A, the direct-current motor 18 is supplied with energy from a battery BI. One terminal of battery BI is connected by a. conductor I20 and switch SW to the motor rotor terminal I22 and one end of the carbon pile resistance element I28 of the control I I0. The other battery terminal is connected by conductor I20 to the motor terminal I24 which is internally connected to one end of the field winding and the rotor. The end of the field winding of motor opposite to that connected to terminal I24 is connected to the terminal of the carbon pile resistance element I28 opposite to that to which the conductor I20 is connected.

In Fig. 2 the resistances R3 and R4 and the end terminals of the transformers T6, T8, and TIO, instead of being connected to the voltage-inducing coils 46 and 41 of the voltage-inducing mechanisms 48, 48A, and 48B, are connected to secondary coils of transformers 200, 202, and 204, respectively. Each of the commutators 206, 200, and 2I0 comprises two substantially semicircular conducting rings C and D insulated from each other. The commutators are arranged relative to each other on the shaft 623 such that the C and D rings sequentially close the circuit to the primary coils of the transformers 200, 202, and 204, for flow of current in the same direction therethrough at 120 intervals during rotation of shaft 623. The reactance of the transformers 200, 202,

and 204 is preferably such that during the time intervals that it is energized, the current through the primary coil continually increases at a substantially constant rate so that the voltage induced in the secondary coils will be of the shape shown in Fig. 5. The C rings of the commutating elements 200, 208, and 2 I0 are connected together and to a slip ring 2 I4 which slip ring is connected to one terminal 01' a battery B2 by a conductor H6. The D rings of the elements 20B. 20B, and 2 I0 are connected together and to a slip ring 2I8 which slip ring is connected to one terminal of the battery B2 by a conductor 220.

The terminals of the primary coils of the transformers 200, 202, and 204 are connected to corresponding brushes E and F diametrically arranged to engage the rings C and D as the shaft 623 rotates, so that for each revolution of the shaft the transformers are energized once with current flowing in one direction and once with current flowing in the opposite direction to bias the grids of the A triodes positively for substantially a half revolution and to bias the B triodes positively for substantially a half revolution, it being remembered that the various transformers are energized at points of revolution of the shaft apart.

The shaft 623 is rotated by a direct current motor [813 similar to motor I0 except that it has a second field winding 2I2. Motor 1813 is connected similarly to the motor 78 to a battery BI with the main field winding connected through the carbon pile resistance element I28 of the control I I8. The second field winding 2l2 is connected between the conductors H6 and H9 so that any unbalance in the voltage drops across the resistors II4 of the system of Fig. 2 and a corresponding parallelly connected system (not shown) identical therewith or a similarly connected resistor II4, as for example, resistor N4 of the systems of Figs. 1A and IE will cause a current to flow through coil 2I2 in directions to momentarily change the angular velocity of the shaft 623 to increase or decrease the voltage phase angles between the parallelly connected systems.

To control the frequency of the output voltage of conductors 42, 66, and 08 and the speed of the motor 188, a circuit for providing current flow of constant magnitude and duration but at variably spaced intervals depending upon the speed of rotation of the motor 18B is provided by a singlephase alternating current generating unit 230 arranged to be driven by the shaft 623. The output terminals of unit 230 are directly connected to the terminals of the primary coil of a saturable core-type transformer having its secondary coil terminals connected directly to the alternating current terminals of a full wave bridge rectifier generally designated 234. The direct current output terminals of the bridge rectifier 234 are connected to the terminals of the control element I ll of the motor speed control H8.

Fig. 8 graphically shows a typical example of the input current curve I, of the flux curve F within the core, and of the secondary current ES of transformer 232. The dotted-line portion of the secondary current, together with the lower solid-line portions of the secondary current ES graphically illustrates the current flow in the D. 0. output system to the control element Ill. As the rotational speed of the shaft 6213 increases the frequency of the voltage generated by the unit 230 increases and the number of current pulses per unit of time likewise increases so that the average current per unit of time increases. This change in average current changes the excitation of the motor 10B and consequently its speed.

In Fig. 3 the grid biasing circuits comprising the grid resistors R3 and R4 and the buses I0, 24, B3, 04, 65, and 61 are fed through transformers 200, 202, and 204 of an inverter of flip flop" network controlled by voltage-inducing mechanisms 46. "A. and 42B driven by a speed-controlled direct-current motor 16A similar to motor 16. The inverter network comprises three pairs of tetrode electronic high vacuum valves 256-261, 266-256, and 266-2" co-operable with the three transformers 266, 262, and 264 respectively. A grounded bus 262 connects together, the cathodes of the valves 266, 258, and 266 and the left-hand end terminals of the voltage-inducing coils 46, 46A, and 46B of the voltage-inducing mechanisms 46, A, and 483. The cathodes of the valves 251, 256, and 26l are connected to a common bus 262 which is selectively connected to the bus 262 by a switch 264 which, during operation, is normally maintained closed but is opened during the starting operation of the network as will be more fully described hereinafter. The valves 256-26l are periodically rendered conductive for energizing the transformers 266, 262. and 266 to control the power valves of groups 6, 2, and ID. The end terminals of the coils 46, 46A, and 26B opposite that to which the bus 262 is connected are respectively connected by means of conductors 265, 266, and 268 to the common terminals of each of the sets of condensers Cl and C2 connected in series between grids of the pairs of valves 256251, 256-459, and 266-26 l. Each of the grids of all the valves 256-26I is connected through a resistor R5 to a common bus 216.

Each pair of valves 256251, 258259, and 266-26l are respectively interconnected together whereby the firing or conducting of one of the valves of any pair will extinguish or render nonconductive the other valve of the pair and whereby the firing of a valve will energize the transformer 266, 262, or 262, with which it is associated. The specific circuit arrangement interconnecting the valves of each pair and connecting each pair to its corresponding transformer are identical, and a specific description of one such circuit arrangement will be sufficient for all three. The anodes of the valves 256 and 251 are connected to opposite end terminals of the primary coil of transformer 266. Plate current is supplied to valves 256 and 251 from a bus 212 of. for example, H5 volts positive D. C. connected to the center tap connection of the primary coil of transformer 266. The zero or negative potential is supplied through the grounded bus 262 to the cathode of valve 256 at all times and by means of switch 264 to the cathode of valves 261, 259, and 261 during normal operation. A resistor R16 is connected across the end terminals of the primary coil of transformer 266 to load the valves 256 and 251 so that charges in loading of the transformer will have little effect on the operation of the valves.

The supplemental anodes of the valves 256 and 251 are connected through resistors RH to the bus 212 to supply the supplemental anode potential. The supplemental anode of valve 256 is connected through an impedance network to the grid of the valve 251 and vice versa so that as one of the valves 256 or 261 conducts, the grid potential of the valve 251 or 256 will be rendered negative to extinguish and maintain the other valve nonconductive and the grid potential of the valve 256 or 251 more positive to maintain the conductive valve conductive. The impedance network comprises a first resistor RH and in series therewith a resistor Hi2 having in parallel therewith a resistor R13 connected in series with a capacitor (H3.

The shaft 620', besides rotating the rotors 69, 26A, and 46B, rotates the rotor of the threephase alternator 62 identical to that shown in the other figures and the alternating current generating unit 226 identical to that shown in Fig. 2. The output of the unit 226 of Fig. 3, however, is utilized in a diiferent manner to control the rotational speed of the shaft 620 in that it is connected to a tuned circuit having a resonating range and a peak resonating frequency within this range. By "resonating range" is meant that range of frequencies wherein the current flowing in the tuned circuit is substantially greater than at frequencies outside this range because of the resonance of the circuit. By "peak resonant frequency" is meant that frequency wherein the current is maximum. The tuned circuit comprises a capacitor CM and an inductive reactor R. connected in series with a full wave bridge rectifier 214. The direct current terminals of the bridge rectifier 214 are connected by conductors 216 and 216 to one of the control windings 266 oi a motor speed-controlling unit 6A which is similar to the unit H6, except that it has the additional winding 266. When the shaft 620 and rotor of the alternator 236 are rotating at the desired speed, the frequency of the voltage supplied to the tuned circuit is, for example, 490 cycles per second. As shown in Fig. 7, this frequency is substantially midway between the beginning of the resonating range and the peak resonant frequency. Therefore, as the frequency of the induced voltage or speed of rotation of the shaft 620 changes, the current flowing through the tuned circuit will increase or decrease, depending upon whether the speed of rotation of the shaft decreases or increases. This changed current flowing through the tuned circuit causes a corresponding change in current flow to the control winding 286 to change the flow of current through the carbon pile resistor I26 for varying the speed of the direct-current motor. The magnitude of current flow through the conductors 216 and 218 is controlled by a variable resistor 262.

The adjustable control element ill of the direct-current speed control IIBA is connected to be responsive to the output of the system of Fig. 3 and is so arranged to provide a slightly drooping characteristic to the motor 16A with increasing output of the system. The control 8A is connected across the direct-current output terminals of the full wave bridge-type rectiher 266 having its alternating current terminals directly connected to the secondary coil of a current transformer 286 connected in the output circuit of one of the system output phases, as for example, conductor 42 of the system of Fig. 3 to provide for a current through the adjustable control element 1 proportional to the current flowing through the conductor 62.

The operation of the systems of Figs. 1A and 1B is as follows: Switch SW of Fig. 1A, is closed to initiate operation of the direct-current motor 18 and the disconnect switch DS is closed to energize the three-phase transformer 4. Energization of the motor 16 acts to rotate the rotors 49, 49A, 49B, and 86, of the voltage-inducing mechanism 48, 46A, and 46B and of the threephase alternator 82. The governor 16 acts to maintain the motor 18 operating at a substantially constant desired speed. The alternator 62 generates a three-phase alternating voltage and if another system such as, for example, theisys- 9 L will go on and of! at the beat frequency of the systems. When the lamps L are substantially out, the switches 98 and LS-l, which may be mechanically interconnected for convenience of operation if desired, are closed, connecting the output of the alternator 82 with the output of the corresponding alternator-B2 of the system of Fig. 1B and connecting the ouput conductors 42, 66, and 68 of the system of Fig. 1A in parallel with the system of Fig. 13 to the output conductors i2, i4 and I6. Switches 9A may also be closed at this time to energize the load dividing circuit.

Voltages are periodically induced in the coils 46 and 41 of the voltage-inducing mechanism 48 which act to periodically bias the grid of the valves Al, A2, and A3 of group 6 positively with respect to their cathodes so that the valves Al, A2, and A3 are placed in condition to conduct current from the alternating current supply conductor I, 2, or 3, to the output transformer T6. Similarly, the voltage induced in the coil 41 of the mechanism 48 will render the valves Bi, B2, and B3 of group 6 in condition for conducting alternately with the A valves. The A valves of group 6, when rendered in a condition to conduct, will select among themselves which is to be the one which actually conducts thecurrent, depending entirely upon the instantaneous relative potentials of the phases of the transformer 4, as is more completely described in my said copending application. Similarly, when the valves Bl, B2, and B3 are rendered by the coil 41 in condition for conducting current these valves will likewise conduct current from the input conductors I, 2 and 3 to the output transformer 'IG, depending upon the instantaneous potentials of the phases of the transformer 4.

Direct current therefore flows periodically through the opposite halves of the center tapped primary coil of the output transformer T6 and returns to the common bus 34 through the con ductor 30 and inductive-reactance 32. Supposing the A valves of group 6 to be conducting and the voltage-inducing mechanism 48 has just rendered the B valves of group 8 conductive, at the first conduction of one of the valves BI, B2, or B3, the potential of the common cathode bus 24 will be raised substantially to the potential of the anode of the conducting one of the valves and since the condenser C6 was charged during the time that the A valves were conducting so thatthe potential thereacross was substantially twice that of the potential between the bus I8 and the conductor 30 and the bus i8 was substantially at the potential of the anode of the conducting one of the A valves, the increase in potential of the bus 24 will immediately transitorily raise the potential of the bus I 8 so that the cathode of the conducting one of the A valves will transitorily be raised to a potential above that of its anode, whereby the conducting valve is extinguished. The capacity of the condenser C6 is such that it will not discharge through its discharge circuit which is the primary coil of the transformer T6 until such time as the conducting one of the valves has had time to deionize.

It will be evident that for each complete revolution of the shaft 62 a potential of the right polarity will be induced in each of the coils 46 and 61 thereof so that the A group of valves will be rendered conductive and the B group of valves will be rendered conductive once during each such revolution. Therefore, the frequency of the voltage in the conductor 42 will be the same frequency as the rotational speed of the shaft 82. The voltage-inducing mechanisms "A and B similarly control the A and 13 valves of groups 8 and I0 whereby alternating current potential is supplied to the conductors 6G and 68 and, as stated hereinbefore, the change in potential of the mechanisms 48, "A and 48B is approximately apart so that the conductors I2, 66 and 88 form a three-phase current output.

Assuming that the systems of Figs. 1A and 1B are now operating in parallel to supply the conductors l2, l4, and [8 with current, the transformers iilii of each system will cause a current flow through the respective resistors H4 which is proportional to the current output of the respective system. Since one terminal of each resistor H4 is grounded, current will flow through conductors H6 and H9 and control elements I II in a direction and of a magnitude proportional to the difference in voltages across the two resistors H4. Suppose, for example, the system of Fig. 1A is carrying more than its share of the load, the voltage drop across the associated resistor I will be greater than the voltage drop across the resistor N4 of the system of Fig. 13. Current will then flow from the resistor N4 of the IA system through the control elements li'l through the resistor H4 01' the IB system and back to the IA system through the ground connection. This current acting through the speed control H8 will cause the motor 18 of Fig. 1A to momentarily decrease its angular velocity and the motor 18 of Fig. 13 to momentarily increase its angular velocity whereby the tants that the valves of the systems of Figs. 1 and 1B are respectively rendered conductive are relatively delayed and hastened so that the current supplied by the system of Fig. 1A is decreased, while the current supplied by the system of Fig. 1B is increased the desired amount so that the two systems will be supplying their correct proportion of current to the output conductors I2, I, and l B.

For synchronizing the two systems, threephase alternators B2 of Figs. 1A and 1B are connected as shown. No power is transferred from either of the alternators to the other when the alternators are dperating at identical speed and are identically phased with respect to each other. Should one of the alternators, however, vary slightly in speed or in phase angle with respect to the other alternator, the leading one of the altemators 82 will tend to drive the lagging one of the alternators 82 as a motor to exert a slight additional torque to the shaft 62 to provide a slightly increased angular velocity thereto. The leading one of the alternators 82 will absorb torque from its driving shaft 62 to impart a slightly decreased angular velocity thereto so that the two systems will tend to remain in absolute synchronous operation with constant phase angles therebetween; it being understood, however, that these changes in speed of the shaft 62 are momentary and merely vary the angular velocity of the two shafts 82 with respect to each other.

In Fig. 2'the grid biasing voltages are provided from a direct-current source such as the battery B2 and applied to transformers 200, 202, and 204 by the commutators 206, 208, and MB, respectively which are rotated by the shaft 623. As the shaft rotates, the conducting portions of the commutators C and D act to alternately energize the primary coil of the associated transformer first in one direction and then in a second direction. As stated hereinbefore, the impedance of 11 the transformers is such that throughout the time interval that the transformer is energized in one direction, the current is constantly increasing to provide a voltage wave in the secondary coils of substantially the form shown in Fig. 5.

It is believed obvious from the above and from the description with respect to the systems of Figs. 1A and 13 that as the motor 168 rotates the commutator, a three-phase alternating voltage will be provided between the conductors 62, 66, and 68 and the current through these conductors will set up a proportional voltage drop across the resistor Ill associated with the threephase rectifying device I68. As the current in the conductors I2, 66, and 66 increases, the voltage drop across the resistor i II will increase, and if a similar resistor Ill connected to another such system has a lesser potential drop thereacross, current will flow through the conductor H6 and through the second fleld cell 212 of the motor "B through the conductor H9 and the corresponding resistor, not shown, of the parallelly connected system back to the ground terminal. This current flow through the coil 2l2 tends to set up a flux which is in the same direction as the flux set up by the normal field coil of the motor 163 to momentarily increase the excitation of the motor 163. This tends to momentarily decrease the angular velocity of the shaft 623 so that the current through the conductors 42, 66, and 68 will be decreased. This current flow through the conductor 6 will tend to have an opposite effect on the motor 16B associated with a parallelly connected system which will tend to set up a flux in the motor which is in opposition to the flux set up by the normal field coil. This results in a, momentary decreased excitation to momentarily increase the angular velocity of the shaft 623 associated therewith so that the associated system (not shown) will tend to take more of the load being supplied to the output conductors I2, l4, and i6.

The shaft 623 also drives the alternating current generating unit 230 whereby a current is caused to flow through the control element ll! of the speed control I It. The transformer 232 is of the saturable core type so that, while the current through the primary winding thereof is continually changing in accordance with the voltage output of the unit 230. the flux which is set up in the core of the transformer 232 merely changes for a. short period each time that the current goes through zero, so that a pulsating current of substantially constant amplitude and duration flows in the secondary circuit of the transformer 232 at a frequency directly proportional to the frequency of the voltage supplied to the transformer by the unit 236. Therefore, as the speed of rotation of the unit 230 increases. more average current will flow through the control element ll'l than would flow were the speed of rotation of the unit 236 lower. This change in current flowing through the element I I1 acts on the carbon pile I26 of the control 8 to vary the current flowing through the normal field coil of the motor 163. As the current flow through the element Ii'l increases, the current flow through the normal field coil will increase, thereby tending to slow down the motor "B. The unit 236 is preferably used to control the actual speed of the shaft 628 and, therefore, the frequency of the voltage supplied by the conductors 42, 66, nd 68, rather than as a means for synchronizing operation between two parallelly connected systems. If a system of Fig. 2 is being operated in parallel with a similar system, the alternator 62 may be connected as described inv connection with the systems of Figs. 1A and 1B; or, if the system of Fig. 2 is to be used by itself, conductors so, 66, and 62 may be connected to a three-phase system operating at the desired frequency and voltage for regulating the speed of the alternator 62 in the same manner. As in the preceding instances the torque capable of being absorbed in or exerted by the alternator B2 is substantially less than that exerted by the motor "B so that the alternator 62 merely serves to slightly increase or decrease the angular velocity of the shaft 623 in order to bring the frequency of the output of conductors I2, 66, and 68, to the exact desired frequency.

In the system shown in Fig. 3 a somewhat more elaborate means for rendering the power valves of the groups 6, 8, and I0 conductive is provided and is operated by suitable voltage inducing mechanisms 46, 46A, and 4813 which have one output coil each 46, 46A, andlGB. At each voltage impulse of the same polarity of the coils of the voltage-inducing mechanisms, one of the two valves associated therewith is rendered conductive and the other non-conductive in alternate fashion; the various groups being rendered conductive and nonconductive at intervals of the revolution of the shaft 620.

In starting the system, the switch 264 is opened and the motor 18A is energized. As the rotors 69, 49A, and 69B rotate, an alternating voltage is periodically induced in the coils 46, 46A, and 668 respectively. At each positive impulse, the grids of the valves 2%, 258, and 260 respectively are rendered positive with respect to their associated cathode and the valves will become conductive and since switch 266 is open will remain conductive. Assume that the switch 264 is closed Just after the mechanism 488 has supplied a positive impulse and before mechanism 48 has started to supply a positive impulse. The next impulse supplied by the coil 46 will place a positive bias on grid of the valve 251 and the valve 251 will be rendered conductive. As soon as the valve 251 becomes conductive, current will flow from the supplemental anode thereof through the resistor Rll which will lower the potential of the end thereof connected to the supplemental anode which will also lower the potential of the control grid of the valve 256. This lowering of the potential of the grid of the valve 256 tends to reduce the flow of current therethrough, reducing the flow of current from the supplemental anode thereof to the cathode thereof, thereby raising the potential of the end of the resistor R" connected to the supplemental anode of the valve 256 which terminal is connected through an impedance network to the control grid of the valve 251. This still further raises the potential of the control grid of the valve 251, causing a greater flow of current therethrough which further lowers the potential of the supplemental anode of the valve 251, and, as before outlined. this will lower the potential of the control grid of the valve 256, causing a further decrease in the current flow therethrough and a consequent raising of the potential of the supplemental anode of the valve 256 and a further raising of the potential of the control grid of valve 251. This series of events happens very rapidly, and valve 256 is soon rendered nonconductive; and the valve 251 rendered fully conductive. A similar series of events occurs at the next pulse of the cell A whereby the valve 258 is rendered conductive and the valve 258 is rendered nonconductive, and, similarly, the next pulse of potential from the coil 45B renders the valve 28I conductive and the valve 258 nonconduetive. The next time a positive impulse is imparted by the mechanlsms 48, 48A and 48B the valves 258, 258 and 260 will be rendered conductive and the valves 251, 259, and 25I rendered nonconductive in the manner hereinbefore described.

When the valve 256 is conducting, plate current flows from the bus 212 through the left-hand half of the primary cell of the transformer 288 through the valve 256 and the bus 262 to the other terminal of the battery or other plate potential supplying means. This flow of current through the left-hand hall of the primary coil of the transformer 208 causes a potential to be induced in the secondary coils of transformer 288 of such polarity that the grids of the A valves of group 6 are rendered positive with respect to their cathodes and the A valve conductive. This same pulse, due to the connections of the right-hand secondary coil, will also tend to render the grids of the B valves of group 8 negative with respect to their cathodes, but this is without elfect, since the valves of the groups 6, 8, and ID are all of the discontinuous control type in that when they once are rendered conductive the grid loses control. As the valve 251 is rendered conductive the valve 256 will become nonconductive, and current will flow from the bus 212 through the right-hand half of the transformer primary coil 208 through the valve 251 to the bus 262. This will cause a voltage of the opposite potential to be induced in the secondary coils so that the grid of the B valves of group 6 will be rendered positive with respect to their associated cathodes and the B valves in condition to conduct. The grids of the A valves of group 6 will be rendered negative with respect to their cathodes, but this is without effect. Assuming the A valves were COIldLlCtil'lg, as hereinbefore set forth, and the B valves now begin to conduct, the commutating action of the condenser C will render the A valves nonconductive as previously described in connection with Fig. 1A. The alternating current generating unit 230, used for frequency control rather than a synchronizer, is driven by the shaft 52C, but in this case, unlike that of the showing in Fig. 2, its output is connected directly into a circuit which is tuned near to the normal output frequency of the unit 230, so that at normal frequency a predetermined current will flow therethrough and through the control winding 288 of the control. unit II8A. This current is of sufilcient magnitude to control the carbon pile resister element I28 so that the proper current will flow through the field winding of the motor 18A. Should, for example, the motor 18A increase in speed for some reason, the frequency of the output voltage of the unit 230 would increase. As shown in Fig. '7, the output current through the control unit 288 would decidedly increase to change the adjustment of the carbon pile element I28 so that the fllOW of current through the field winding of the motor 18A would be increased to cause the motor 18A to slow down somewhat. Likewise, should the speed of the motor 18A be lowered for some reason, the current flowing through the control winding 288 would be reduced, causing the carbon pile element I28 to decrease the flow of current to the field winding of the motor 18A, causing the motor to increase in 14 speed. Due to the fact that the output circuit of the unit 230 is a resonant circuit with the normal frequency supplied thereto substantially midway between the beginning of the resonating range and the peak resonant frequency, as defined hereinbefore, slight changes in output frequency of the unit 230 will cause substantial changes in current flow through the control winding 288 and consequently close control of the speed of the motor "A.

In Fig. 3 the three-phase current transformer I00 has been omitted and a single-phase current transformer 288 has been utilized instead, it being assumed that there will be no great degree or unbalance in the current flow of the three conductors 42, 66, and B8. The output of the current transformer 288 is rectified and supplied as direct current to the control winding III of the control IIBA. Increase in current flow through the conductor 42 will increase proportionately the flow of current through winding III tending to increase the flower current through carbon pile I28, decreasing slightly the speed of rotation of the motor 18A to provide a slightly drooping characteristic for the motor 18A with respect to an increased output of the system.

The rotors or rotating members 49, 48A, 49B, and are shown to comprise two poles. If it is desired to decrease the speed of the shafts on which the members are mounted, they may comprise four, six, or eight poles, as may be required. The conducting portions or segments of the commutators 286, 288, and 2I8 are shown to be two in number, as indicated by the reference characters C and D. If it is desired to reduce the speed of the shaft 623, these segments may be increased to four, six, or eight, as may be desired.

Although not shown in the drawing, the load to be supplied to any one of my improved devices is connected to conductors I2, I4, and IB. It is obvious that the operating frequency of the device is completely independent of the magnitude of the current drawn by the load. The frequency determining elements, such as the motor I8, receive their excitation from sources independent of the input or output systems.

In accordance with the patent statutes, preferred embodiments of the invention have been described in detail. However, it will be appreciated that in the broad aspect of the invention various modifications in the form, number, and arrangement of the parts, as well as a recombination of the systems and control units therefor shown in the various figures may be made without departing from the spirit and scope of the invention, which is to be limited only by the scope of the appended claims.

What is claimed and is desired to be secured by United States Letters Patent is as follows:

1. In an electrical control system for transferring energy from an electrical input system to an electrical output system, the speed of rotation of said rotary means being independent of the magnitude of said load in said output system, electric valve means interconnecting said systems, rotary means for rendering said valve means conductive in predetermined succession, and means for rotating said rotary means proportional to the desired frequency of said output system.

2. In an electrical control system for transferring energy from anelectrical input system to an electrical output system having a load, a plurality of electric valve means, each means comprising an anode and a cathode and a grid, circult means connecting said anodes to said input system, and said cathodes to said output system, circuits for said grids. rotary means for energizing said grid circuits in predetermined succession whereby each of said grids is periodically rendered at a potential with respect to its associated cathode whereby said valve means are periodically rendered in condition for flow of current therethrough. and means for rotating said rotary means proportional to the desired frequency of said output system, the speed of rotation of said rotary means being independent of the magnitude of said load in said output system.

3. In an electrical control system for transferring energy from an electrical input system to an electrical output system having a load, a plurality of electric valve means. each means comprising an anode and a cathode and a g id, circuit means connecting said anodes to said input system, and said cathodes to said output system. circuits for said grids, rotary means for energizing said grid circuits in predetermined succession whereby each of said grids is periodically rendered at a potential with respect to its associated cathode whereby said valve means are alternately rendered in condition for flow of current therethrough, means for rotating said rotary means proportional to the desired frequency of said output system, and means responsive to variations in speed of said rotary means from the desired speed for regulating the speed of rotation of said rotary means, the speed of rotation of said rotary means being independent of the magnitude of said load in said output system.

4. In an electrical control system for transferring energy from an electrical input system to an electrical output system having a load, a plurality of electric valve means, each means comprising an anode and a cathode and a grid, circuit means connecting said anodes to said input system, and said cathodes to said output system, circuits for said grids, impulse regulating means for said grid circuits, said impulse means being so constructed and arranged relative to said grid circuits as to provide a voltage-time wave shape in said grid circuit of substantially rectangular form, rotary means for controlling said impulse regulating system whereby said grid circuits are energized in predetermined succession for periodically rendering each of said grids at a potential with respect to its associated cathode whereby said valve means are alternately rendered in condition for flow of current therethrough, and means for rotating said rotary means proportional to the desired frequency of said output system, the speed of rotation of said rotary means being independent of the magnitude of said load in said output system.

5. In an electrical control system for transferring energy from an electrical input system to an alternating current electrical output system, a plurality of electric valve means interconnecting said systems, rotary means for controlling the conductivity of said valve means and the frequency of alternations of said output system, a tuned circuit having a resonating frequency range, voltage generating means responsive to the speed of rotation of said rotary means and operable at the predetermined desired rotative speed of said rotary means to generate an alternating voltage within the resonant range of said tuned circuit whereby when the rotative speed of said rotary means varies from said desired speed said generating means will generate an alternating voltage at a diflerent frequency with- 16 in the resonant frequency range of said tuned circuit, circuit means connecting the output of said generating means to said tuned circuit, and means responsive to current flow in said tuned circuit for altering the speed of rotation of said rotary means.

6. In an electrical control system for transferring energy from an electrical input system to an alternating current electrical output system, a plurality of electric valve means interconncting said systems, rotary means for controlling the conductlvityof said valve means and the frequency of alternations of said output system, a tuned circuit having a resonating frequency range with a peak resonant frequency, voltage generating means responsive to the speed of rotation of said rotary means and operable at the predetermined desired rotative speed of said rotary means to generate an alternating voltage within the resonant range of said tuned circuit but at a frequency other than said peak resonant frequency whereby when the rotative speed of said rotary means varies from said desired speed said generating means will generate an alternating voltage at a different frequency within the resonant frequency range of said tuned circuit, circult means connecting the output of said generatin means to said tuned circuit, and means responsive to the change in current flow in said tuned circuit for altering the speed of rotation of said rotary means in a direction toward said predetermined desired speed.

7. In an electrical control system for transferring energy from an electrical input system to an alternating electrical output system, a plurality of electric valve means interconnecting said systems. rotary means for controlling the conductivity of said valve means, a tuned circuit having a resonating frequency range, voltage generating means responsive to the speed of rotation of said rotary means and operable at the predetermined desired rotative speed of said rotary means to generate an alternating voltage within the resonant range of said tuned circuit whereby when the rotative speed of said rotary means varies from said desired speed said generating means will generate an alternating voltage at a different frequency within the resonant frequency range of said tuned circuit, rectifying means for said tuned circuit, circuit means connecting the output of said generating means to said tuned circuit, and means responsive to rectifled current flow in said tuned circuit for altering the speed of rotation of said rotary means.

8. In an electrical control system for transferring energy from an electrical input system to an electrical output system, a plurality of electric valve means, each means comprising an anode and a cathode and a grid, circuit means connecting said anodes to said input system, and said cathodes to said output system, circuits for said grids, rotary means for energizing said grid circuits whereby said grids are alternately biased for alternately rendering said valve means in condition for flow of current therethrough, means for rotating said rotary means proportional to the desired frequency of said output system, a tuned circuit resonant to a predetermined frequency, voltage generating means responsive to the'speed of rotation of said rotary means and operable at a predetermined desired rotative speed of said rotary means to generate an alternating voltage having a. frequency slightly differing from the resonant frequency of said tuned circuit whereby when the rotative speed of said rotary means changes lromsaiddesiredspeedsaidsenerating rotary means.

9. In an electrical control system for transferring energy from an electrical input system to an electrical output system, a plurality of electric valve means, each means comprising an anode and a cathode and a grid, circuit means connecting said anodes to said input system, and said cathodes to said output system, circuits for said grids, rotary means for energizing said grid circuits whereby said grids are alternately biased for alternately rendering said valve means in condition for flow of current therethrough, an electric motor having a speed control mechanism as sociated therewith for rotating said rotary means proportional to the desired frequency of said output system, a tuned circuit having a resonating frequency range, voltage generating means responsive to the speed of rotation of said rotary means and operable at the predetermined desired rotative speed of said rotary means to generate an alternating voltage within the resonant range of said tuned circuit whereby when the rotative speed of said rotary means changes from said desired speed said generating means will generate an alternating voltage at a diil'erent frequency within the resonant frequency range of said tuned circuit, rectifying means for said tuned clrcuit, circuit means connecting the output of said generating means to said tuned circuit, and means responsive to rectified current flow in said tuned circuit for regulating said speed mechanism whereby the speed of rotation of said rotary means is altered in a direction toward said predetermined speed.

10. In an electrical control system for transierring energy from an electrical input system to an electrical output system, a plurality of electric valve means, each means comprising an anode and a cathode and a grid, circuit means connecting said anodes to said input system, and said cathodes to said output system, circuits for said grids, rotary means for energizing said grid circuits whereby said grids are alternately rendered at a potential with respect to the associated cathode whereby said valve means are alternately rendered in condition for flow of current therethrough, means for rotating said rotary means proportional to the desired frequency of said output system, voltage generating means having an ou p t l rn ting voltage of a frequency proportional to the speed of rotation of said rotary means, and means responsive to the frequency of said generating means for providing a pulsating control current for said rotating means of constant magnitude and of a frequency proportional to the frequency of said alternating voltage.

11. In an electrical control system for transferring energy from an electrical input system to an electrical output system, a plurality of electric valve means, each means comprising an anode and a cathode and a grid, circuit means connecting said anodes to said input system. and said cathodes to said output system. circuits for said grids, rotary means for energizing said grid circuits whereby said grids are alternately rendered at a potential with respect to the associated cathode whereby said valve means are alternately rendered in condition for how of current there- Mini p through, means for rotating said rotary mean proportional to the desired frequency of said out put system. voltage generating means having at output alternating voltage of a frequency proportional to the'speed of rotation of said rotary means. a saturating core transformer having or input circuit connected to said generating meam and having an output circuit, rectifying meam connected to said output circuit and operable tc supply a pulsating direct current of constant magnitude and of a frequencyrproportionate to the frequency of said generating means voltage. and means responsive to said pulsating direct current for regulating the speed of said rotary means.

12. In an electrical control system for transferring electrical energy from a polyphase electrical input system to a polyphase electrical output system having a load, a plurality of pairs of electrical valve means interconnecting-said systems, a voltage inducing mechanism for alternately rendering conductive each of the valve means of the pairs of valve means, a common means for rotating said mechanisms, means controlling thespeed of rotation of said common rotating means whereby each of said valve means is rendered in condition to conduct once each cycle of the output system, the speed of rotation of said common rotating means being independent of the magnitude of said load in the output ystem.

13. In a system for supplying'a multiphase alternating current output circuit from a source of electrical energy. the combination of a pinrality of translating units individual to said phases and common to said source, each unit including means actuable to translate energy from the source to single-phase alternating current and delivering the same to a corresponding phase of said output circuit, means for supplying a periodic voltage to said units comprising 'a rotatable means having an output circuit for'each of said phases. means connecting said rotating means output circuits to said units whereby each unit is operable to receive a periodic voltage impulse each cycle of said output circuit, and means for rotating said rotatable means at a predetermined s 14. A system for supplying a multiphase alternating current output circuit by a pair of interconnecting networks from a source of electrical energy, in which each network comprises a pinrality of translating units individual to said phases and common to said source, each unit including means actuable to translate energy from the source to single-phase alternating current and delivering the same to a corresponding phase of said output circuit, means for supplying a periodic voltage to said units comprising a rotatable means having an output circuit for each of said phases. means connecting said rotating means output circuits to said units whereby each unit is operable to receive a periodic voltage impulse each cycle of said output circuit, means for rotating said rotatable means at a predetermined speed, and a polyphase enerating means rotatable by said rotating means: and circuit means interconnecting said generating means whereby said networks are maintained in synchronism.

15. A system for supplying a multiphase alternating current output circuit by a pair of interconnecting networks from a source of electrical energy, in which each network comprises a pinrality of translating units individual to said phases and common to said source, each unit including means actuable to translate energy from the source to single-phase alternating current and delivering the same to a corresponding phase oi said output circuit, means for supplying a periodic voltage to said units comprising a rotatable means having an output circuit for each or said phases, means connecting said rotating means output circuits to said units whereby each unit is operable to receive a periodic voltage impulse each cycle of said output circuit. means for regulating the speed of rotation oi said rotatable means and circuit means for producing a potential proportional to the output of said units; and circuit means interconnecting said speed regulating means and said potential producing circuit means whereby the phase anfles of each nework will be relatively adjusted to proportion the load between said networks.

16. A system for supplying a multiphase alternating current output circuit by a pair of interconnecting networks irom a source of electrical energy, in which each network comprises a pinrality oi translating units individual to said phases and common to said source, each unit including means actuable to translate energy from the source to single-phase alternating current and delivering the same to a corresponding phase of said output circuit, means for supplying a periodic voltage to said units comprising a rotatable means having an output circuit for each of said phases, means connecting said rotating means output circuits to said units whereby each unit is operable to receive a periodic voltage impulse each cycle of said output circuit, means for regulating the speed of rotation of said rotatable means. a circuit including rectifying means and a. resistor connected for flow oi rectified current, and means for causing a flow of current in said last-named circuit proportional to the current flow through said units: and circuit means interconnecting said regulating means and said resistors whereby the phase angles of each network will be relatively adjusted to proportion the current flow from said source to said output circuit between said networks.

1'1. In an electrical control system for transierring electrical energy from a polyphase electrical input system to a polyphase electrical output system, a plurality 01! pairs of electrical valve means, each oi said pairs being connected to a difi'erent phase of said output system, each of said valve means having anode means connected to certain phases oi said input system and a common grid connection and common cathode connection, circuit means connecting the common cathode connections of said pairs of valve means to a phase of said output system, a voltage in- 20 ducing mechanism ior each oi the pairs of common grid connections 101' each of said pairs of valve means. a common means for rotating said mechanisms, means controlling the speed of rotation oi said common rotating means whereby each oi said common grid connections is rendered at a potential relative to that oi the associated common cathode connection for placing each valve means in condition to conduct once each cycle of the output system. a

18. In an electrical control system for transierring electrical energy from a polyphase electrical input system to a polyphase electrical output system, a plurality of pairs of electrical valve means, each of said pairs being connected to a different phase at said output system, each of said valve means having anode means connected to certain phases of said input system and a common grid connection and common cathode connection, circuit means connecting the common cathode connections of said pairs of valve means to a phase oi said output system. a voltage inducing mechanism for each of the pairs oi common grid connections for each 0! said pairs oi valve means, said inducing mechanism being operable to generate a substantially rectangular voltage wave shape, a common means for rotating said mechanisms, means controlling the speed of rotation of said common rotating means whereby each of said common grid connections is rendered at a potential relative to that of the associated common cathode connection for placing each valve means in condition to conduct once each cycle of the output system.

19. In an electrical control system for transierring energy from an electrical input system to an electrical output system having a load. electric valve means interconnecting said systems, rotary means for rendering said valve means conductive in predetermined succession independently of said input system, and means for rotating said rotary means proportional to the desired frequency of said output system, the speed of rotation of said rotary means being independent 01' the magnitude of said load in the output system.

PAUL T. NIMS.

REFERENCES CITED The following references are of record in the me of this patent:

UNITED STATES PATENTS Number Name Date 1,954,661 Alexanderson et ai. Apr. 10, 1934 1,971,188 Kramer Aug. 21, 1934 Certificate of Correction Patent No. 2,447,134. August 17, 1948.

PAUL T. NIMS It is hereby certified that errors appear in the n'nted specification of the above numbered patent requiring correction as follows: olumn 7, line 55, for charges read changes; column 19, line 16, for nework read network; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 2nd day of November, D. 1948.

THOMAS F. MURPHY,

Assistant Umnmiuiomr of Patents. 

